refactor: move act/model and act/exprparser to actions-proto-go

Gitea needs the workflow model and the expression evaluator to parse
workflows and to build the task payload this runner consumes, so it had
to depend on gitea.com/gitea/runner for it. Both packages now live in
gitea.dev/actions-proto-go, the module both sides already share, and this
repository consumes them from there.

The GithubContext helpers that need a git checkout on disk (SetRef,
SetSha and SetRepositoryAndOwner) are not moved: they are runner only and
would drag a git client and the act context logger into the shared
module. They become functions of the new act/ghcontext package, together
with their tests.

act/common.CartesianProduct moved to the shared model package as well,
act/model was its only user. act/model/testdata/container-volumes is now
act/runner/testdata/container-volumes, its only user is runner_test.go.

The x-runner-uuid and x-runner-token header names come from
actions-proto-go too, so the runner and Gitea cannot drift apart.

Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>

internal/pkg/client/header.go

@@ -3,7 +3,11 @@
 
 package client
 
+import "gitea.dev/actions-proto-go/pkg/protocol"
+
+// The headers are defined in the shared protocol package so that Gitea and the
+// runner cannot drift apart.
 const (
-	UUIDHeader  = "x-runner-uuid"
-	TokenHeader = "x-runner-token"
+	UUIDHeader  = protocol.UUIDHeader
+	TokenHeader = protocol.TokenHeader
 )